1. Field of the Invention
The present invention relates to a receiver circuit comprising an equalizer that monitors intersymbol interference and whose characteristics are controlled to suppress the intersymbol interference and, more particularly, to a receiver circuit that is adapted to allow suitable equalization of a received waveform with a high data rate by means of an equalizer.
2. Description of the Related Art
In cases where signal transmission between LSI chips, signal transmission between elements in a chip or between circuit blocks, or signal transmission between circuit boards, or the like, is performed at high speed, Intersymbol Interference (ISI) occurs in the transmission lines, which produces distortion in the received waveform. When the transmission line bandwidth is not adequately high enough in comparison with the data rate, ISI arises due to loss of the high frequency component of the signal waveform and affects the signal waveform at a past or future time such that the impulse response of the received waveform has a low foot. This effect is ISI. That is, the ISI of a past or future pulse signal appears in a received waveform at a certain time that has propagated through the transmission line. Due to the occurrence of this ISI, the transmittable/receivable transmission distance is shortened or the transmittable/receivable data rate is reduced. Hence, compensation for ISI in high-speed signal transmission is required.
Such suppression of the ISI of a received signal is generally also performed in a reception/playback circuit of communication, for example, as shown in Japanese Patent Application Laid Open No. H6-204911 (disclosed on Jul. 22, 1994), for example. This conventional technology is provided with a root-cosine rolloff filter in order to suppress ISI.
Further, an adaptive equalizer is also illustrated in Japanese Patent Applications Laid Open Nos. 2003-059186 and 2003-045121, for example.
In order to compensate the ISI of a transmission line, an equalizer that provides an inverse characteristic for the transmission line is disposed on the transmission side or reception side of the transmission line or on both sides thereof. By providing an equalizer on the transmission side of the transmission line to wave-shape (equalize the waveform of) the transmitted pulse signal, waveform distortion caused by ISI is not produced on the reception side, whereby the ISI of the transmission line can be suppressed to a certain degree. However, when the characteristic of the transmission line varies with time, for example, simply providing an equalizer with a fixed characteristic on the transmission side is insufficient for ISI compensation. When the transmission line characteristic fluctuates, an equalizer must be provided on the reception side, the output of the equalizer must be monitored and the characteristic of the equalizer must be minutely adjusted to establish the inverse characteristic of the transmission line. As a result, even when fluctuations of the transmission line characteristic occur, optimal equalization can be performed on the reception side by adapting to such fluctuations.
One possibly considered method for adjusting the characteristic of the equalizer provided on the reception side is to generate an ISI evaluation function that indicates the degree of the ISI from the output of the equalizer to control the characteristic parameters of the equalizer to minimize the ISI. With this method, the ISI evaluation function must be generated by sampling the signal waveform at the data center timing substantially at the center of a data eye that comprises a plurality of received signal waveforms in which variation is present. When the data rate is low, the sample timing adjustment to the center timing of the data eye can be performed relatively easily but, as the data rate increases, the cycle (unit interval: UI) of the signal waveform grows shorter and, therefore, highly accurate adjustment of the sample timing adjustment to the center timing of the data eye is problematic. Hence, it is difficult to determine an ISI evaluation function at an appropriate sample timing and therefore difficulties with suitable ISI compensation using the equalizer characteristic adjustment method above are to be expected.